Optical sensor for detecting chemical substances dissolved or dispersed in water

ABSTRACT

A highly sensitive optical sensor in a simple structure for detecting a chemical substance dissolved or dispersed in water is provided. The optical sensor ( 1 ) comprises a detecting element ( 2 ) having a polymer thin film, a light source unit ( 3 ) for emitting light for irradiating the polymer thin film, and a photodetector ( 4 ) for detecting the intensity of light reflected from the polymer thin film. The detecting element ( 2 ), the light source unit ( 3 ), and the photo-detector ( 4 ) are integrally mounted in a housing. The polymer thin film is formed on a highly reflective substrate or an optically transparent substrate so as to interact with a chemical substance dissolved or dispersed in water flowing through a water channel ( 8 ).

The present invention relates generally to an optical sensor and moreparticularly to an optical sensor which utilizes a polymer thin film fordirectly detecting a chemical substance dissolved or dispersed in water,particularly dissolved organic carbon (hereinafter abbreviated as “DOC”)in accordance with an optical detecting method such as a waveguide modemethod (WG method), surface plasma resonance method (SPR method),interference enhanced reflection method (IER method), and so on. Thepolymer thin film interacts with a chemical substance such ashydrocarbon and so on which may be absorbed into or adsorbed on thepolymer thin film. As a result, the polymer thin film exhibits a changein thickness and/or refractive index depending on the concentration ofthe chemical substance, so that such a physical change may be measuredby an optical method to determine the concentration of the chemicalsubstance dissolved or dispersed in water, particularly, DOC.

A variety of reports have been made on the use of polymer thin films inoptical sensors for detecting chemical species in gas phase and chemicalsubstances dissolved in water. Many of these reports are related tofiber optic sensors or optical waveguide sensors based on evanescentwaves or guided waves.

As is well known in the art, when a light beam is incident on aninterface between two dielectric materials having different refractiveindices n₁ and n₂ (>n₁), respectively, total internal reflection occurswhen the light beam is incident from the dielectric material of therefractive index n₁ to the dielectric material of the refractive indexn₂ and when the angle of incidence is larger than a critical angle θc.The critical angle θc of total internal reflection is given by:

θc=sin⁻¹ (n₁/n₂)  (1)

In this case, the incident light is fully reflected back into thedielectric material of the refractive index n₂ so that no light willenter the dielectric material of refractive index n₁. However, thereexists a wave function called an evanescent wave which propagates inparallel with the interface between the dielectric material ofrefractive index n₁ and the dielectric material of the refractive indexn₂. The electric field E of the evanescent wave decays exponentiallywith the distance z from the interface, and can be expressed by anexponential function:

E=E₀exp(−z/d_(p))  (2)

where E₀ is the electric field on the interface, and d_(p) is the depthof penetration defined as the distance where the electric field of theevanescent wave, produced when light is incident on the interface at anangle θ, is reduced from the value at the interface to 1/e, and isexpressed by:

d_(p)=λ[2π(n₂ ²sin² θ−n₁ ²)^(½)]  (3)

As is well known in the art, optical waveguides operate based on theprinciple of total internal reflection. A planar waveguide, which is onetype of the optical waveguides, simply consists of a first medium ofrefractive index n₂ sandwiched between a second medium of refractiveindex n₁ and a third medium of refractive index n₃, where the refractiveindices of the media are selected such that n₂>n₃≧n₁ is satisfied. Alight beam is confined in the first medium by successive totalreflections when the light beam is traveling in the medium n₂ at anangle θ larger than the critical angle of total internal reflection oninterfaces of the first medium and the two other media (in this event,sin θ>n₃/n₂≧n₁/n₂ is satisfied). In this case, waveguiding occurs, andthe light waves existing in the first medium are called guided waves.Optical fibers are another type of waveguides consisting of acylindrical core of refractive index n₂ surrounded by a cladding layerof refractive index n₁ (<n₂).

In either evanescent wave sensors or guided wave sensors, light musttravel at an angle larger than the critical angle of total internalreflection. Typical examples of optical chemical sensors based onevanescent waves can be found in many prior art documents. Carter et al.disclose in USP No. Re.33064 a method of identifying a chemical speciesin a solvent using an optical waveguide covered with a response filmhaving a refractive index smaller than that of the waveguide layer.Light propagates through the optical waveguide by the action of totalinternal reflection.

Within the propagating light, evanescent waves generated by the totalreflection only are involved in interaction of the response film with achemical species under detection. Thus, the method proposed by Carter etal. is limited only on interaction which is accompanied with absorptionor scattering of light, or generation of fluorescence.

Hinrich et al. have reported the use of polymer for detecting organiccompounds in water on an internal reflection element in “Determinationof organic compounds by IR/ATR spectroscopy with polymer-coated internalreflection elements” (Applied Spectroscopy, Vol. 44, No. 10, 1990, pp1641-1646). However, the detecting method of Hinrich et al. relies onthe absorption of evanescent waves of infrared rays penetrating in thepolymer film by organic compounds, wherein the polymer film is used toeliminate water and extract the organic compounds on the surface of theinternal reflection element to thereby enhance an absorption signal.

Burck et al. have reported a similar method except for the use of anoptical fiber in “A fiber optic evanescent field absorption sensor formonitoring organic contamination in water” (Fresenius J. Anal. Chem.,(1994), 342, pp 394-400) and “Fiber-optic evanescent wave sensor for insitu determination of non-polar organic compounds in water” (Sensors andActuators, B 18-19 (1994), pp 291-295).

Japanese Laid-open Patent Application No. 7-85122 (1995) discloses amethod for detecting an organic solvent in water with an optical fiberhaving a cladding layer made of a chitosan compound. Since the intensityof evanescent waves penetrating into the chitosan cladding layer dependson the degree of swelling, and the concentration of the chitosancladding layer varies in accordance with the ratio of water to solvent,the intensity of light propagating the optical fiber is consequently afunction of the concentration of organic solvent dissolved in water.

A main disadvantage of a sensor utilizing evanescent waves, however, isthat the sensitivity of the sensor is limited since only a portion ofincident light is used for detection. Thus, a long interaction distanceis required to realize a sensor having a high sensitivity. This imposesa limit on reduction in size of such sensors.

A larger portion of incident light may be utilized for detection toprovide sensors having higher sensitivities. WO95/20151 discloses achemical sensor having a multi-layered optical fiber. Specifically, asensing polymer layer is sandwiched between a core of the optical fiberand a cladding layer, and the refractive index of the polymer layer islarger than that of the cladding layer so that the polymer layer servesas an optical waveguide layer. With this structure, light incident tothe optical fiber is refracted toward the polymer waveguide layer andpropagates therethrough toward the end terminal of the sensor. However,since this structure requires an output light detector to be locatednear an output terminal, the chemical sensor disclosed in WO95/20151 isinconvenient for measuring a substance to be detected in water.

A large number of highly sensitive polymers for detecting chemicalsubstances in gas have also been reported. Gliliani et al. have reporteda strip-shaped polymer waveguide having a thickness of 1 μm fordetecting the existence of several kinds of organic vapors in“Fabrication of an integrated optical waveguide chemical vapormicrosensor by photopolymerization of a bifunctional oligomer” (Appl.Phys. Lett., 48 (1986), pp 1311-1313) and “Integrated optical chemicalvapor microsensor” (Sensors and Actuators, 15 (1988), pp 25-31). Amethod proposed by them involves introducing non-polarized light into awaveguide channel from one end of an optical fiber by fiber coupling,and extracting the light propagating through the waveguide from theother end to the outside. In this way, interaction between the polymerand an organic vapor is sensed as a change in intensity of thetransmitting (propagating) light. The method of Gluliani et al.,however, implies the following two difficulties: (1) a photopolymerizedpolymer is required to fabricate the strip-shaped waveguide; and (2) thestrip-shaped waveguide must be coupled by end-fire coupling to a thinfilm having a thickness on order of micrometers.

A planar polymer thin film optical waveguide as a sensor for detectingorganic vapors has been reported by Bowman and Burgess in “Evaluation ofpolymer thin film waveguides as chemical sensors” (SPIE Proceedings,Vol. 1368: Chemical, biochemical, and environmental II, 1990). Thepolymer film exhibits a change in waveguide characteristic as a resultof absorbing chemical vapors. Bowman et al. use two gratings(diffraction gratings) embedded in a substrate for coupling incident anddecoupling light. Such a grating coupler, however, is difficult tofabricate and expensive. A similar method using a simpler prism couplerhas been reported by Osterfeld et al. in “Optical gas detection usingmetal film enhanced leaky mode spectroscopy” (Appl. Phys. Lett. 62 (19),1993, pp 2310-2312). A metal reflective layer is sandwiched between apolymer film waveguide and an optical coupling prism such that lightincident to the interface between the metal reflective layer and theprism is totally reflected at an optimal incident angle. Evanescentwaves produced by total reflection excite a waveguide mode in thepolymer film.

Reference has not been made as to whether or not the polymer waveguidesproposed by Bowman et al. and Osterfeld et al. can be used for detectingorganic carbon in water. Further, since the polymer film (teflon AF)used by Osterfeld et al. nas a refractive index (=1.3034) smaller thanthe refractive index (=1.33) of water, the film made of teflon AF doesnot function as a waveguide in water.

Optical sensors without using evanescent waves or guided waves have alsobeen reported. For example, Gauglitz et al. have reported a method ofreflection spectroscopy for detecting organic vapors using the swellingof polymer films (GIT Fachz. Lab., 889, 7/1990). In this method, asensitive polymer thin film coated on a transparent substrate isirradiated with white light at a normal incidence from the substrateside, and reflected light from the polymer thin film is collected andanalyzed by a spectrometer. Here, wavelength shift in the reflectionspectra caused by polymer-vapor interaction is measured as an indicationof organic vapor concentration. As will be later described, the normalincident arrangement is less sensitive and must therefore rely onspectral interferometry. In other words, the method of Gauglitz et al.is complicated and requires expensive and large equipment forimplementation.

The present invention has been made to solve the foregoing problems ofthe known techniques, and its object is to provide an optical sensor fordetecting a chemical substance dissolved or dispersed in water,particularly DOC, which is simple in structure, highly sensitive, andeasy to fabricate.

To achieve the above object, the present invention provides an opticalsensor for directly detecting a chemical substance dissolved ordispersed in water comprising:

at least one detecting element having a polymer thin film capable ofinteracting with the chemical substance;

at least one light source unit for emitting light for irradiating thepolymer thin film; and

a first photo-detector for detecting the intensity of light reflectedfrom the polymer thin film.

In the present invention, while the polymer thin film is capable ofdetecting any chemical substance which is absorbed or adsorbed thereby,the polymer thin film is preferably used to detect organic carbon interms of the sensitivity and so on.

The detecting element, the light source unit, and the photo-detector areintegrally supported by a housing. The polymer thin film provided in thedetecting element is preferably formed on a planar substrate. In oneembodiment of the present invention, the polymer thin film is formed ona highly reflective substrate such as that made of silicon, metal or thelike, and interaction between the polymer thin film and the organiccarbon in water is detected in accordance with an IER method (so calledthe front-side IER, abbreviated as FS-IER). In another embodiment of thepresent invention, the polymer thin film is formed on an opticallytransparent substrate, with a light source and a photodetector beinglocated on the substrate side, i.e., facing the side of the substrate onwhich the polymer thin film is not formed, and interaction between thepolymer thin film and the organic carbon in water is detected inaccordance with the IER method (so called the back-side IER, abbreviatedas BS-IER). In another embodiment of the present invention, the polymerthin film is formed on a highly reflective metal layer deposited on atransparent substrate. The highly reflective metal layer has a thicknessequal to or less than a wavelength of light from the light source unit,and is made of a material selected from a group including silver, gold,chrome, silicon, and germanium. In this embodiment, interaction betweenthe polymer thin film and organic carbon in water is detected inaccordance with one of a SPR method and a WG method.

The polymer thin film preferably has a thickness of 10 μm or less, morepreferably 5 μm or less, and further preferably 3 μm or less. Desirably,the light source unit comprises a laser diode (LD) or a light emittingdiode (LED), and the photo-detector is a photodiode or aphototransistor. An output of the photo-detector is applied to anelectric circuit which generates a signal indicative of theconcentration of a chemical substance in water.

In the present invention, the polymer thin film absorbs or adsorbs achemical substance in water to directly respond to the chemicalsubstance. As a result of such interaction, the polymer thin filmexhibits a change in thickness and/or refractive index. Since such aphysical change is related to the concentration of the chemicalsubstance, the physical change can be measured in accordance with anoptical approach such as an IER method, SPR method, waveguide modemethod, or the like to derive the concentration of the chemicalsubstance in water.

In the four optical methods, the concentration of the chemical substancein water may be measured as a function of the intensity of reflectedlight at a fixed detection angle. With the waveguide mode method, theconcentration of the chemical substance is measured as a function of thereflectivity of the polymer thin film or an angular position of awaveguide mode.

In the FS-IER method, a light source and a photodetector are locatedabove the polymer thin film such that probe light from the light sourceand reflected light from the polymer thin film pass through water inwhich organic substances are dissolved. This method is not alwaysdesirable because bubbles and particles in water may scatter or blocklight beams, causing large fluctuations or attenuation of output signal.

The BS-IER method takes an approach similar to that reported by Gauglitzet al., which employs a sensing element having a polymer thin filmformed on an optically transparent substrate, with a light source and aphotodetector being located on the substrate side, i.e., facing the sideof the substrate on which the polymer thin film is not formed. Theinventors of this invention, however, have found that when probe lightis incident at an angle less than but close to the critical angle oftotal internal reflection, the reflectivity of the polymer thin filmlargely varies as compared with light incident normal to the polymerthin film, as reported by Gauglitz et al. This invention has been madebased on this discovery and provides an optical sensor different fromconventional evanescent wave sensors and guided wave sensors.

In the BS-IER method, the substrate couples light from the light sourceunit to the polymer thin film at a predetermined angle, and functions aslight coupling means for coupling light reflected by the sensing elementto the first light detector, and the predetermined incident angle is setat a value smaller than a critical angle of total internal reflection onthe interface between the polymer thin film and the water and close tothe critical angle.

In one embodiment of this invention, the optical sensor furthercomprises a second light detector for directly receiving light from thelight source unit, and an electronic circuit to receive outputs of thefirst light detector and the second light detector for calculating theratio of these outputs to generate a signal indicative of theconcentration of the organic substance. The light source unit, the firstlight detector, the second light detector, and the light coupling meansare mounted in a housing in a predetermined positional relationship withrespect to the sensing element.

Also, the substrate may be a prism or a planar plate. When the substrateis a planar plate, a grating may be formed on a predetermined positionof the substrate, or a grating layer formed with a grating may bedisposed between the substrate and the polymer thin film or on a surfaceof the substrate on which the polymer thin film is not formed.

FIG. 1A generally illustrates the FS-IER structure of a first embodimentof an optical sensor according to the present invention;

FIG. 1B is an enlarged cross-sectional view illustrating the structureof a detecting element in FIG. 1A;

FIG. 2 is a graph representing the relationship between the thicknessand the reflectivity of a polymer thin film in the optical sensorillustrated in FIG. 1A;

FIG. 3 is a schematic diagram illustrating a basic configuration of theBS-IER of a sensing element for use in an optical sensor according tothis invention;

FIG. 4 is a graph illustrating the relationship between an incidentangle of light and the reflectivity of a polymer thin film in thesensing element of FIG. 3;

FIG. 5 is a graph illustrating the relationship between the thickness ofa polymer thin film and its reflectivity in the sensing element of FIG.3 for a plurality of incident angles;

FIG. 6A is a cross-sectional view generally illustrating an embodimentof the optical sensor according to this invention;

FIG. 6B is an enlarged view illustrating in detail a portion of theoptical sensor of FIG. 6A;

FIG. 7 is a graph illustrating the relationship between the thickness ofa polymer thin film and its reflectivity in the optical sensor of FIG. 6for a plurality of substrates having different refractive indices;

FIGS. 8A-8E are cross-sectional views for explaining exemplarymodifications to the sensing element in the optical sensor according tothis invention, where FIGS. 8A-8D illustrate sensing elements utilizinggrating coupling, and FIG. 8E illustrates a sensing element utilizingside-coupling;

FIG. 9A generally illustrates the structure of a second embodiment ofthe optical sensor according to the present invention;

FIG. 9B is an enlarged cross-sectional view illustrating the structureof a detecting element in FIG. 9A;

FIG. 10 is a graph representing the relationship between an incidentangle of light emitting on the polymer thin film in the optical sensorof FIG. 9A and the reflectivity of the polymer thin film;

FIG. 11 is a graph representing the relationship between an incidentangle of light emitting on the polymer thin film in the optical sensorof FIG. 9A and the reflectivity of the polymer thin film when thepolymer thin film is forced to respond to 2-ppm toluene, for showing ashift of a resonance coupling angle of a waveguide mode TM₄;

FIG. 12 is a graph representing the relationship between an incidentangle of light and the reflectivity of the polymer thin film derived fora specific example of the optical sensor illustrated in FIG. 9A;

FIG. 13 is a graph showing how a response of an optical sensor havingthe same structure as the optical sensor of FIG. 9A varies over time for4-ppm toluene in water;

FIG. 14 is a graph showing how a response of an optical sensor havingthe same structure as the optical sensor of FIG. 9A varies for a varyingconcentration of toluene in water;

FIG. 15 is a graph showing how a response of an optical sensor havingthe same structure as the optical sensor of FIG. 9A varies over time for20-ppm toluene in water; and

FIG. 16 is a graph showing how the reflectivity of a polymer thin filmused in an optical sensor having the same structure as the opticalsensor of FIG. 1 changes in response as the concentration of toluene inwater varies.

FIG. 17 is a graph illustrating a change in response time of the opticalsensor according to this invention as a function of time when an organicsubstance in water is toluene having a concentration of 10-200 ppm;

FIG. 18 is a graph illustrating that the response of the optical sensoraccording to this invention linearly changes as the concentration oftoluene in water varies; and

FIG. 19 is a graph illustrating that the response of the optical sensoraccording to this invention linearly changes as the concentrations oftoluene, benzene, and p-xylene in water vary.

A certain kind of polymer thin film (later described) exhibits a changein thickness and/or refractive index when a chemical substance such asorganic carbon or the like is absorbed into or adsorbed on the polymerthin film. The present invention measures such a physical change of thepolymer thin film to sense a chemical substance in water. Severalembodiments of an optical sensor according to the present invention willhereinafter be described with reference to the accompanying drawings. Itshould be noted that in the drawings, the same or similar components aredesignated by the same reference numerals, and repetitive explanationthereof will be omitted.

FIG. 1A generally illustrates the configuration of a first embodiment ofan optical sensor according to the present invention, and FIG. 1Billustrates in an enlarged view a detector element used in the opticalsensor. The sensor of the first embodiment relies on the FS-IER methodfor detecting a chemical substance in water. Referring specifically toFIG. 1A, the optical sensor 1 comprises the detecting element 2; a lightsource unit 3 for emitting light such that the light is incident to thedetector element 2 at an incident angle θ; and a first photo-detector 4for detecting the intensity of light emitted from the light source unit3 and reflected by a polymer thin film 2 ₂ of the detecting element 2.

The detecting element 2 is positioned on one surface of a base 5, andhas a planar reflective substrate 2 ₁ and the polymer thin film 2 ₂formed on the substrate 2 ₁ in contact with water, as illustrated inFIG. 1B. The reflective substrate 2 ₁ is preferably a substrate having ahigh reflectivity and may be, for example, mirror, semiconductor, metal,or a thin film of any metal material or any semiconductor materialdeposited on a low reflective substrate.

The light source unit 3 has a light source 31, a beam splitter 32, and apolarizing plate 33. The light source 31 may be a laser diode (LD) or alight emitting diode (LED) for emitting visual light or infrared rays.Light emitted from the light source 31 is split by the beam splitter 32into two portions, one of which is polarized by the polarizing plate 33and emits on the polymer thin film 2 ₂ of the detecting element 2. Lightreflected from the polymer thin film 2 ₂ enters, through a window 41,the first photo-detector 4 which detects the intensity of receivedlight. The other light portion split by the beam splitter 32 is directedto a second photo-detector 6 and transduced thereby into a signalrepresentative of a reference light intensity.

The first photo-detector 4 and the second photo-detector 6 may bephotodiodes or phototransistors. Outputs of these photo-detectors 4, 6are transferred to an appropriate electronic circuit to calculate theratio of the output of the first photo-detector 4 to the output of thesecond photo-detector 6. This ratio is used to generate a signalindicative of the concentration of a chemical substance to be detected.

The light source unit 3, the first photo-detector 4, and the secondphoto-detector 6 are mounted at appropriate locations in a housing 7.The housing 7 is mounted on the base 5 such that the housing 7 forms awater channel 8 with the base 5 and the polymer thin film 2 ₂ of thedetecting element 2 is in contact with water in the water channel 8. Thelight passing through the polarizing plate 33 is preferably s-polarizedlight which has an electric field vector of the light perpendicular toan incident plane of the polymer thin film 2 ₂.

The IER method is utilized to detect a change in thickness and/orrefractive index of the polymer thin film 2 ₂ in contact with waterbased on the fact that the intensity of light reflected from a thindielectric film depends on the thickness of the dielectric film. Thepolymer thin film 2 ₂ exhibits a change in thickness and/or refractiveindex when it absorbs or adsorbs a chemical substance in water. Thus,when the polymer thin film 2 ₂ is irradiated with light from the lightsource unit 3, a change in thickness and/or refractive index of thepolymer thin film 2 ₂ appears as a change in intensity of lightreflected from the polymer thin film 2 ₂. It is therefore possible tomeasure the concentration of a chemical substance in water by measuringthe intensity of the reflected light.

FIG. 2 illustrates a graph which represents the relationship between thethickness of the polymer thin film 2 ₂ of the detecting element 2 in theoptical sensor 1 shown in FIG. 1A and the reflectivity of the polymerthin film 2 ₂ to s-polarized light incident thereto, when the polymerthin film 2 ₂ is formed on a silicon substrate and placed in water. Inother words, the graph shows reflectivity curves derived in accordancewith the IER method. A solid line indicates the reflectivity when theincident angle θ of the light is 80, and a broken line indicates thereflectivity when the incident angle θ is 70. In this event, therefractive index of the polymer thin film is 1.50.

Although the thickness of the polymer thin film 2 ₂ may be arbitrarilyselected in a range of several nanometers (nm)-10 micrometers (μm), thethickness is desirably set at a value away from minimum values of thereflectivity curves in FIG. 2 in order to appropriately detect thethickness of the polymer thin film 2 ₂ in accordance with the IERmethod. Also, it can be seen from the reflectivity curves of FIG. 2 thatthe reflectivity more largely modulates as the incident angle θ islarger (the reflectivity exhibits a larger change in response to achange in thickness). Thus, the incident angle θ is preferably 70 ormore.

FIG. 3 generally illustrates a basic configuration of the BS-IER of asensing element for use in the optical sensor according to thisinvention. Referring specifically to FIG. 3, a sensing element 51comprises a transparent substrate 52 and a polymer thin film 53 formedon one surface of the substrate 52 by spin coating or the like. Thepolymer thin film 53 has a thickness d and a refractive index n₂. Otherthan the spin coating, the polymer thin film 53 may be formed by any ofgenerally known methods such as vapor deposition, dip coating, rollercoating, sputtering, chemical vapor deposition (CVD), and so on. Assumethat the surface of the polymer thin film 53, opposite to the substrate52, is in contact with water having a refractive index n₃. A lightsource 54 for emitting linearly polarized monochromic light ofwavelength λ is located opposing the substrate 52. Monochromic light 55emitted from the light source 54 is incident on the substrate 52 at anangle θ and reflected by an interface 56 between the polymer thin film53 and the water and by an interface 57 between the polymer thin film 53and the substrate 52, respectively. Thus, the intensity of reflectedlight from the sensing element 1 is a combination of the intensity oflight 58 reflected by the interface 56 and the intensity of light 59reflected by the interface 57, and therefore is the sum or difference ofthe intensities of the light 58, 59 depending on optical path lengths ofthe respective light 58, 59.

The reflectivity of the sensing element 51 illustrated in FIG. 3 can becalculated using the well-Known Fresnel formula. As to further detailson the Fresnel formula, see “Principles of Optics”, by M Born and E.Wolf, Pergmon Press, 1959. Assume herein that a sensing element isformed of an SF11 glass substrate having a refractive index n₁ equal to1.7786 and a poly(octadecyl methacrylate-co-glycidyl methacrylate) thinfilm (hereinafter referred to as “poly(ODMA-co-GLMA) thin film), havinga thickness d equal to 1.8 μm and a refractive index n₂ equal to 1.493,coated on one surface of the glass substrate. The sensing element islocated such that the poly(ODMA-co-GLMA) thin film is in contact withwater having a refractive index n₃ equal to 1.332, with the exposedsurface of the SF11 glass substrate irradiated with p-polarized lightand s-polarized light of wavelength λ equal to 632.8 nm. Then, thereflectivity of the poly(ODMA-co-GLMA) thin film is calculated in themeasuring conditions mentioned above as a function of the incident angleof the light on the substrate. FIG. 4 illustrates the results of thecalculations. As illustrated in the graph of FIG. 4, the critical angleof total internal reflection θ_(c23) is equal to 48.495° for theinterface between the poly(ODMA-co-GLMA) thin film and the water. Forreference, the critical angle of total internal reflection λ_(c12) (notshown) is equal to 57.079° for the interface between the SF11 glasssubstrate and the poly(ODMA-co-GLMA) thin film.

It can be seen from the graph of FIG. 4 that when the incident angle θof light is larger than the critical angle θ_(c23) (=48.495°), thereflectivity of the poly(ODMA-co-GLMA) thin film to the s-polarizedlight (TE wave) and the p-polarized light (TM wave) is unity, and doesnot at all depend on the thickness of the poly(ODMA-co-GLMA) thin film,and the reflectivity of the poly(ODMA-co-GLMA) thin film to thes-polarized light and the p-polarized light strongly depends on thethickness of the poly(ODMA-co-GLMA) thin film when the incident angle θis smaller than the critical angle θ_(c23).

FIG. 5 illustrates the relationship between the thickness of thepoly(ODMA-co-GLMA) thin film and the reflectivity of the same tos-polarized light when the sensing element used for deriving the graphof FIG. 4 is irradiated with light of the same wavelength (λ=632.8 nm)for a plurality of incident angles smaller than the critical angleθ_(c23) (=48.495°). Comparison between reflectivity curves calculatedwith different incident angles illustrated in the graph of FIG. 5reveals that the reflectivity of the poly(ODMA-co-GLMA) thin filmstrongly depends on its thickness as the incident angle θ approaches thecritical angle θ_(c23) when the incident angle θ is smaller than thecritical angle θ_(c23), and that the dependency of the reflectivity ofthe poly(ODMA-co-GLMA) thin film on its thickness is largest when theincident angle θ is around 48. The depth of modulation is abruptlyreduced as the incident angle θ is smaller than the critical angleθ_(c23). At the incident angle θ equal to 10, the depth of modulationbecomes extremely small.

It will be apparent from the above discussion that the normal incidentarrangement used by Gauglitz et al., as described previously, has anextremely small change by the thickness or refractive index of a polymerthin film, and is therefore not sensitive. One embodiment of thisinvention is intended to provide an optical sensor which has a highsensitivity in a simple structure.

FIG. 6A is a cross-sectional view generally illustrating the BS-IERconfiguration of an optical sensor according to the present invention.The optical sensor 10 comprises a sensing element 11, a light sourceunit 12, a first light detector 13, and a second light detector 14. Ascan be best seen in an enlarged view of FIG. 6B, the sensing element 11comprises a polymer thin film 16 formed on one surface of a prism 15,acting as the substrate 2 of FIG. 3, in a predetermined thickness. Theprism 15 is mounted on a flow cell 17 for passing water therethroughsuch that the polymer thin film 16 is in contact with water flowingthrough the flow cell 17. The flow cell 17 has a flow inlet 18 and aflow outlet 19 for water.

The light source unit 12 comprises a light source 20, a beam splitter21, and a polarizing plate 22. The light source 20 may be, for example,a laser diode (LD) or a light emitting diode (LED) which emits visiblelight or infrared rays. Light emitted from the light source 20 is splitby the beam splitter 21 into a probe beam and a reference beam. Theprobe beam passes through the polarizing plate 22 and becomes a linearlypolarized beam. The polarization of this linearly polarized beam ispreferably an S-polarization (i.e., the electric field of the light beamis oriented perpendicular to the plane of incident). The probe beampasses through the prism 15, is incident on the polymer thin film 16 atan incident angle θ smaller than a critical angle of total internalreflection θon the interface between the polymer thin film 16 and water,and is reflected by the polymer thin film 16. The reflected probe beamis received by the first light detector 13 which transduces the probebeam into an electric signal indicative of the intensity of thereflected light. The reference beam, which is the other light beam splitby the beam splitter 21, is received by the second light detector 14which transduces the received light beam into an electric signalindicative of a light intensity for reference. The signals outputtedfrom the first light detector 13 and the second light detector 14 aresupplied to an appropriate electronic circuit having, for example, asampling hold circuit, a comparator, and so on, for calculating theratio between these signals. The concentration of an organic substancein water can be determined using this ratio.

For implementing the emission, reflection, and detection of light beams,the light source unit 12, the first light detector 13, and the secondlight detector 14 are mounted in a housing 23 in a predeterminedpositional relationship with respect to the sensing element 11 asillustrated in FIG. 6A. Alternatively, optical fibers may be used tocouple between the light source 12 and the prism 15 and between theprism 15 and the first light detector 13. Photodiodes andphototransistors may be used as the first light detector 13 and thesecond light detector 14, by way of example.

In the optical sensor according to the BS-IER method, the incident angleθ of light emitted from the light source unit is desirably smaller thanthe critical angle of total internal reflection θon the interfacebetween the polymer thin film and water and close to the critical angleθc. By selecting the incident angle θ such that the reflectivity of thepolymer thin film of zero thickness, is preferably 0.1 or more,particularly preferably 0.2 or more, and further preferably 0.3 or more,more highly sensitive sensors can be provided. For example, asillustrated in the graph of FIG. 5, s-polarized light having thewavelength at 632.8 nm is preferably incident at an incident angle θranging from 40° and 48° on a poly(ODMA-co-GLMA) thin film spin-coatedon an SF11 glass substrate. On the other hand, while it is desirablethat the thickness of a polymer thin film is generally in a range ofseveral nanometers to 10 μm, the thickness of a polymer thin filmoptimal to the detection in accordance with the IER method is notlimited in particular. However, as will be understood from the graph ofFIG. 5, since a change in reflectivity is small near a maximum value ora minimum value of the reflectivity, the thickness of the polymer thinfilm is desirably selected in the middle of a thickness at which thereflectivity is maximum and a thickness at which the reflectivity isminimum.

The substrate for use in the sensing element of the optical sensoraccording to the BS-IER method is preferably transparent and may be madeof materials including, for example, glass, plastic, polymer andsemiconductor. In addition, an extremely thin metal layer, inorganicdielectric film or semiconductor film (50 nm or less) may be vapordeposited on such a transparent substrate. It should be noted, however,that the reflectivity of a polymer thin film varies depending on therefractive index of a material used for the substrate. FIG. 7 is a graphshowing how the refractive index of a substrate influences thereflectivity of a polymer thin film. For the purpose of measurements,three sensing elements are prepared. Specifically, a poly(ODMA-co-GLMA)thin film having a thickness d equal to 1.8 μm and a refractive index n₂equal to 1.493 is formed by spin-coating on one surface of each of threetransparent substrates having a refractive index n1equal to 1,5143,1.7786 and 2.3513, reflectively. Each of the sensing elements ispositioned such that the poly(ODMA-co-GLMA) thin film is in contact withwater having a refractive index n₃ equal to 1.332. The graph shows therelationship between the thickness of the polymer thin film and itsreflectivity when each substrate is irradiated with a light beam havinga wavelength equal to 632.8 nm. It can be seen from the graph that asubstrate having a larger refractive index allows the reflectivity ofpolymer thin film to be more sensitive to a change in its thickness, andaccordingly is desirable for use in the optical sensor.

Continuing the explanation on the substrate, the prism 15 of the sensingelement 11 in the optical sensor illustrated in FIG. 6A acts as a lightcoupling means for coupling the probe beam from the light source unit 12to the polymer thin film 16 and for coupling a reflected beam therefromto the first light detector 13 as well as constitutes the substrate 2for forming the polymer thin film 16 thereon. However, such a lightcoupling means is not limited to the prism but may be realized by avariety of other means. Typical examples of such coupling means may bethose utilizing grating coupling and side-coupling.

In the following, sensing elements utilizing the grating coupling willbe described with reference to FIGS. 8A-8D, and a sensing elementutilizing the side-coupling with reference to FIG. 8E. FIG. 8Aillustrates a sensing element which has a grating 25 on a portion of asurface of a transparent substrate 24 and a polymer thin film 26 coatedon the surface having the grating 25 formed thereon. A sensing elementillustrated in FIG. 8B has a grating layer 28 formed with a grating 27positioned between a substrate 24 and a polymer thin film 26. A sensingelement illustrated in FIG. 8C is formed with a grating 25 on a surfaceof a substrate 24 in a portion on which light is incident and in aportion from which light reflected by a polymer thin film 26 exits. FIG.8D illustrates a sensing element which employs a grating layer 28 formedwith a grating 27 on a surface, on which light is incident, and mountedon a surface of a substrate 24 opposite to a polymer thin film 26. Asensing element illustrated in FIG. 8E, in turn, utilizes theside-coupling such that light is detected to be incident on one sidesurface 29 perpendicular to a polymer thin film 26 on a substrate 24 andreflected light from the polymer thin film 26 is led out through theother side surface 30.

FIG. 9A generally illustrates the configuration of a third embodiment ofthe optical sensor according to the present invention, and FIG. 9B is anenlarged cross-sectional view illustrating the structure of a detectingelement shown in FIG. 9A. The third embodiment differs from the secondembodiment of FIG. 6A in that the second embodiment uses a polymerwaveguide formed on a metal cladding layer.

Referring specifically to FIGS. 9A and 9B, a metal layer 2 ₃ isdeposited on the bottom 91 of a prism 9, and a polymer thin film 2 ₂serving as a polymer waveguide is formed on the metal layer 2 ₃ tocomplete a detecting element 2. The prism 9 is mounted on a flow cell 17having a water flow inlet 18 and a water flow outlet 19 such that thepolymer thin film 2 ₂ faces water passing through the flow cell 17.Light emitted from a light source unit 3 and polarized by a polarizingplate 33 is incident to the bottom 91 of the prism 9 at an angle largerthan an internal total reflection angle of the prism 9, and theintensity of light reflected by the bottom 91 is measured by a firstphoto-detector 4. The metal layer 2 ₃ has a thickness equal to or lessthan the wavelength of the light emitted from a light source 31, andpreferably made of silver, gold, chrome, silicon, or germanium.

When the light emitted from the light source 31 is totally reflected bythe bottom 91 of the prism 9, evanescent waves are produced and lightwaves in a waveguide mode are excited by the evanescent waves. Suchexcitation of the waveguide mode in the polymer thin film 2 ₂ i.e.,optical coupling is the strongest at the incident angle at which atangential component of an evanescent wave vector on the bottom 91 ofthe prism 9 is equal to a wave vector of the light waves in thewaveguide mode. Thus, under such a condition, the energy of incidentlight from the light source 31 transitions to light waves in thewaveguide mode internal to the polymer thin film 2 ₂, whereby theintensity of light reflected from the metal layer 2 ₃ is abruptlydecreased.

Thus, when the reflectivity of the polymer thin film 2 ₂ is measuredwhile varying the incident angle θ of the light from the light source31, excitation of light waves in the waveguide mode can be recognized asabrupt attenuation of a curve representing the reflectivity at a certainresonance coupling angle. FIG. 10 is a graph of values measured in anexperiment for showing changes in reflectivity of the polymer thin film2 ₂ with respect to an incident angle θ of light emitting on thedetecting element 2 comprising a poly(ODMA-co-GLMA) layer of 2 μm inthickness. In the curve illustrated in FIG. 10, four waveguide modesTM₁, TM₂, TM₃, TM₄ can be recognized. The poly(ODMA-co-GLMA) layer maybe spin coated on the surface of a gold layer having a thickness ofapproximately 50 nm vapor-deposited on the bottom of a rectangular prismmade of SF11 glass (the refractive index of which is 1.7780 atwavelength of 632.8 nm). Since the poly(ODMA-co-GLMA) layer has arefractive index of approximately 1.46 in water, which is larger thanthose of water and gold, the poly(ODMA-co-GLMA) layer functions as awaveguide.

When the polymer thin film 2 ₂ responds to a chemical substance inwater, i.e., absorbs or adsorbs the chemical substance, the polymer thinfilm 2 ₂ exhibits a change in thickness and/or refractive index to causea shift of a resonance coupling angle at which a certain waveguide modeis excited. Such a shift of angle is a function of the concentration ofthe chemical substance. Thus, the concentration of a chemical substancein water can be sensed by measuring a shift of the resonance couplingangle associated with a certain waveguide mode. FIG. 11 is a graphrepresenting a shift of the resonance coupling angle associated with thewaveguide mode TM₄ (FIG. 10) when the polymer thin film 2 ₂ responds totoluene in concentration of 2 ppm, resulted in a change δ ofreflectivity. Specifically, a solid line represents the relationshipbetween an incident angle of light emitting on the polymer thin film 2 ₂and the reflectivity of the polymer thin film 2 ₂ when the concentrationof toluene is 0 ppm, while a broken line represents the relationshipbetween the incident angle and the reflectivity after the polymer thinfilm 2 ₂ has responded to toluene in concentration of 2 ppm.

Instead of measuring a shift of the resonance coupling angle, theconcentration of a chemical substance in water can be sensed by fixingan incident angle θ of light from the light source 31 at one side of thewaveguide mode resonance and measuring a change in intensity of lightreflected from the detecting element 2. Since the resonance is quitesharp, even a very small shift of the resonance coupling angle appearsas a large change in reflectivity.

For supporting at least one waveguide mode, the polymer thin film 2 ₂illustrated in

FIG. 9A must have a sufficient thickness. For example, a cutoffthickness of the polymer thin film 2 ₂ having a refractive index of 1.45in water is approximately 284 nm in order for the polymer thin film 2 ₂to have a TEO mode. When the thickness of the polymer thin film 2 ₂ isequal to or less than the cutoff thickness, any waveguide mode cannotexist in the polymer thin film 2 ₂. It is however possible to observe adifferent phenomenon referred to as “surface plasmon resonance”(hereinafter abbreviated as “SPR”).

The surface plasmon is plasma oscillation of free electrons existing onthe boundary of a metal. This plasma oscillation is affected by therefractive index of a substance proximate to a surface of a metal. Forexample, when p-polarized light is incident to the bottom 91 of theprism 9 in the optical sensor constructed as illustrated in FIG. 9A andevanescent waves are produced by internal total reflection, surfaceplasma oscillation can be excited. The plasma oscillation is excited atan incident angle θ at which a tangential component of an evanescentwave vector on the bottom 91 of the prism 9 matches a wave vector ofplasma waves on an interface opposing to the polymer thin film 2 ₂(i.e., on the interface with the substrate) with respect to the metallayer 2 ₃. In this event, the energy of the incident light istransferred to plasma waves to cause the intensity of reflected light toabruptly attenuate. This phenomenon is the surface plasmon resonance(SPR). The position of the resonance coupling angle of SPR largelydepends on the refraction of the polymer thin film on the surface of themetal layer, so that the SPR method may also be unitized to sense achemical substance in water.

When the reflectivity to total internal reflected light is measuredwhile varying an incident angle θ of incident light, the SPR can beexperimentally observed as abrupt attenuation of the reflectivity at acertain resonance coupling angle. As an example of observed SPR, FIG. 12illustrates a reflectivity curve (SPR curve) showing how thereflectivity of the polymer thin film used in the optical sensorillustrated in FIG. 9A varies with respect to an incident angle 6 whenthe optical sensor comprises a poly(ODMA-co-GLMA) layer having athickness of 107 nm which is spin coated on the surface of a gold layerhaving a thickness of 50 nm vapor-deposited on the bottom of arectangular prism made of SF11 glass (the refractive index of which is1.7780 at wavelength of 632.8 nm).

Another optical sensor having the structure illustrated in FIG. 9A andrelying on the SPR method can also be realized. While this opticalsensor is also irradiated with p-polarized light, the polymer thin filmshould be as thin as possible, and preferably has a thickness in a rangebetween several nanometers and several hundreds of nanometers. This isbecause the surface plasmon is a surface phenomenon and is sensitive toa change which may occur at a position several nanometers to severalhundreds of nanometers away from the surface of the metal layer. Whenthe polymer thin film absorbs or adsorbs a chemical substance in waterand swells to cause a change in thickness and/or refractive index, theresonance coupling angle of SPR is shifted. It is therefore possible tosense the concentration of the chemical substance in water by measuringa shift of the resonance coupling angle of SPR or by measuring a changein intensity of light reflected from the polymer thin film with anincident angle of light from the light source being fixed at the nearresonance coupling angle of SPR.

Materials for the polymer thin film used in the optical sensor accordingto the present invention preferably include a homopolymer or copolymerhaving a recurring unit represented by the following chemical formula(I):

where X represents —H, —F, —Cl, —Br, —CH₃, —CF₃, —CN, or —CH₂—CH₃;

R¹ represents —R² or —Z —R²;

Z represents ——, —S—, —NH—, —NR²—, —(C═Y)—, —(C═Y)—Y—, —Y—(C═Y)—,—(SO₂)—, —Y′—(SO₂)—, —(SO₂)—Y′—, —Y′—(SO₂)—Y′—, —NH—(C═O)—, —(C═O)—,—(C═O)—NH—, —(C═O)—NR²′—, —Y′—(C═Y)—Y′—, or —O—(C═O)—(CH₂)n—(C═O)—O—;

Y represents the same or different O or S;

Y′ represents the same or different O or NH;

n represents an integer ranging from 0 to 20; and

R² and R²′ represent the same or different hydrogen, a linear alkylgroup, a branched alkyl group, a cycloalkyl group, an un-saturatedhydrocarbon group, an aryl group, a saturated or un-saturated heteroring, or substitutes thereof. It should be noted that R¹ does notrepresent hydrogen, a linear alkyl group, or a branched alkyl group.

In the formula, X is preferably H or CH₃; R¹ is preferably a substitutedor non-substituted aryl group or —Z—R²; Z is preferably —O—, —(C═O)—O—,or —O—(C═O)—; R² is preferably a linear alkyl group, a branched alkylgroup, a cycloalkyl group, an un-saturated hydrocarbon group, an arylgroup, a saturated or un-saturated hetero ring, or substitutes thereof.

A polymer used as the polymer thin film 2 ₂ for the present inventionmay be a polymer consisting of a single recurring unit (I), a copolymerconsisting of another recurring unit and the above-mentioned recurringunit (I), or a copolymer consisting of two or more species of therecurring unit (I). The recurring units in the copolymer may be arrangedin any order, and a random copolymer, an alternate copolymer, a blockcopolymer or a graft copolymer may be used by way of example.Particularly, the polymer thin film is preferably made frompolymethacrylic acid esters or polyacrylic acid esters. The side-chaingroup of the ester is preferably a linear or branched alkyl group, or acycloalkyl group with the number of carbon molecules ranging preferablyfrom 4 to 22.

Polymers particularly preferred for the polymer thin film are listed asfollows:

poly(dodecyl methacrylate);

poly(isodecyl methacrylate);

poly(2-ethylhexyl methacrylate);

poly(2-ethylhexyl methacrylate-co-methyl methacrylate);

poly(2-ethylhexyl methacrylate-co-styrene);

poly(methyl methacrylate-co-2-ethylhexyl acrylate);

poly(methyl methacrylate-co-2-ethylhexyl methacrylate);

poly(isobutyl methacrylate-co-glycidyl methacrylate);

poly(cyclohexyl methacrylate);

poly(octadecyl methacrylate);

poly(octadecyl methacrylate-co-styrene);

poly(vinyl propionate);

poly(dodecyl methacrylate-co-styrene);

poly(dodecyl methacrylate-co-glycidyl methacrylate);

poly(butyl methacrylate);

poly(butyl methacrylate-co-methyl methacrylate);

poly(butyl methacrylate-co-glycidyl methacrylate);

poly(2-ethylhexyl methacrylate-co-glycidyl methacrylate);

poly(cyclohexyl methacrylate-co-glycidyl methacrylate);

poly(cyclohexyl methacrylate-co-methyl methacrylate);

poly(benzyl methacrylate-co-2-ethylhexyl methacrylate);

poly(2-ethylhexyl methacrylate-co-diacetoneacrylamide);

poly(2-ethylhexyl methacrylate-co-benzyl methacrylate-co-glycidylmethacrylate);

poly(2-ethylhexyl methacrylate-co-methyl methacrylate-co-glycidylmethacrylate);

poly(vinyl cinnamate) poly(butyl methacrylate-co-methacrylate);

poly(vinyl cinnamate-co-dodecyl methacrylate);

poly(tetrahydrofurfuryl methacrylate);

poly(hexadecyl methacrylate);

poly(2-ethylbutyl methacrylate);

poly(2-hydroxyethyl methacrylate);

poly(cyclohexyl methacrylate-co-isobutyl methacrylate);

poly(cyclohexyl methacrylate-co-2-ethylhexyl methacrylate);

poly(butyl methacrylate-co-2-ethylhexyl methacrylate);

poly(butyl methacrylate-co-isobutyl methacrylate);

poly(cyclohexyl methacrylate-co-butyl methacrylate);

poly(cyclohexyl methacrylate-co-dodecyl methacrylate);

poly(butyl methacrylate-co-ethyl methacrylate);

poly(butyl methacrylate-co-octadecyl methacrylate);

poly(butyl methacrylate-co-styrene);

poly(4-methyl styrene);

poly(cyclohexyl methacrylate-co-benzyl methacrylate);

poly(dodecyl methacrylate-co-benzyl methacrylate);

poly(octadecyl methacrylate-co-benzyl methacrylate);

poly(benzyl methacrylate-co-benzyl methacrylate);

poly(benzyl methacrylate-co-tetrahydrofurfuryl methacrylate);

poly(benzyl methacrylate-co-hexadecyl methacrylate);

poly(dodecyl methacrylate-co-methyl methacrylate);

poly(dodecyl methacrylate-co-ethyl methacrylate);

poly(2-ethylhexyl methacrylate-co-dodecyl methacrylate);

poly(2-ethylhexyl methacrylate-co-octadecyl methacrylate);

poly(2-ethylbutyl methacrylate-co-benzyl methacrylate);

poly(tetrahydrofurfuryl methacrylate-co-glycidyl methacrylate);

poly(styrene-co-octadecyl acrylate);

poly(octadecyl methacrylate-co-glycidyl methacrylate);

poly(4-methoxystyrene);

poly(2-ethylbutyl methacrylate-co-glycidyl methacrylate);

poly(styrene-co-tetrahydrofurfuryl methacrylate);

poly(2-ethylhexyl methacrylate-co-propyl methacrylate);

poly(octadecyl methacrylate-co-isopropyl methacrylate);

poly(3-methyl-4-hydroxystyrene-co-4-hydroxystyrene);

poly(styrene-co-2-ethylhexyl methacrylate-co-glycidyl methacrylate);

It should be noted that in the methacrylate ester polymers or copolymerslisted above, acrylate may be substituted for methacrylate. The polymersmay be crosslinked on their own, or they may be crosslinked byintroducing a compound that has crosslinking reactive groups. Suitablecrosslinking reactive groups include, for example, an amino group, ahydroxyl group, a carboxyl group, an epoxy group, a carbonyl group, anurethane group, and derivatives thereof. Other examples include maleicacid, fumaric acid, sorbic acid, itaconic acid, cinnamic acid, andderivatives thereof. Materials having chemical structures capable offorming carbene or nitrene by irradiation of visible light, ultravioletlight, or high energy radiation may also be used as crosslinking agents.Since a film formed from crosslinking polymer is insoluble, the polymerforming the polymer thin film of the sensor may be crosslinked toincrease the stability of the sensor. The crosslinking method is notparticularly limited, and methods utilizing irradiation of light orradioactive rays may be used in addition to known crosslinking methods,for example, a heating method.

It should be noted that in the optical sensor according to thisinvention, the refractive index of the polymer thin film and swelling ofthe polymer thin film vary with temperature, so that the characteristicsof the optical sensor are inevitably affected somewhat by ambienttemperature. For this reason, temperature control techniques ortemperature compensation techniques are required for highly accuratemeasurements. Specifically, such temperature control and temperaturecompensation may be implemented by appropriate techniques including: (1)arrangement of the optical sensor in a temperature controlled housing orcontrol of the temperature of water flowing through the flow cell; (2)creation of a temperature compensating signal using a temperaturesensitive element attached on the optical sensor; (3) utilization of apolymer more sensitive than the polymer thin film to temperature tocreate a reference signal for temperature correction; and so on.

Further, the optical sensor according to this invention may be modifiedto allow for measurements of the concentration of a mixture of organicsubstances (for example, hydrocarbons) dissolved in water. Suchmodifications may be realized by one or a combination of the followingoptions:

(1) a method using a kind of polymer thin film which has a low or smallresponse to a plurality of kinds of chemical substances in water;

(2) a method using a plurality of polymer thin films, each of which isselectively responsive to a different kind of chemical substance inwater, wherein responses derived from these polymer thin films arecombined to output a signal indicative of the concentration of a mixturecomposed of a plurality of chemical substances in water;

(3) a method using a plurality of polymer thin films, each of whichexhibits a different response to the same or different chemicalsubstance in water, wherein responses derived from these polymer thinfilms are combined to output a signal indicative of the concentration ofa mixture composed of a plurality of chemical substances in water; and

(4) a method using one or a plurality of the above-mentioned polymerthin films in combination of different optical approaches (for example,the IER method, surface plasmon resonance, guided wave modespectroscopy), to generate a signal indicative of the concentration of amixture composed of a plurality of chemical substances in water.

It should be noted however that a plurality of sensing elements, lightsource units, and/or light sensing elements may be required depending onparticular implementations using one or a combination of the foregoingapproaches.

Actually, the sensitivity and response time of the optical sensoraccording to this invention may vary depending on the kind of organicsubstance to be measured. Thus, in order to accurately measure the totalamount of organic substances and the concentrations of individualorganic substances or groups of organic substances in a variety ofapplications, a multi-channel configuration of optical sensors may beconfigured using the methods (1)-(4) mentioned above, in combination ofknown pattern recognition techniques (for example, matrix analysis,neural network analysis, and so on).

Several examples of the optical sensor according to the presentinvention will be described below.

EXAMPLE 1

FIG. 13 is a graph showing how a response of an optical sensor havingthe structure illustrated in FIG. 9A varies over time when theconcentration of toluene in water is 4 ppm. An employed detectingelement comprised a poly(ODMA-co-GLMA) layer of 2 μm in thickness whichwas spin coated on a gold layer of 50 nm in thickness deposited on thebottom of a rectangular prism made of SF11 glass. A laser diode emittinglight at wavelength of 670 nm was used for a light source. Light fromthe laser diode was split into a reference beam and measuring beam. Thereference beam was directed to a silicon photodiode for reference, whilethe measuring beam was passed through a polarizing plate and directed tothe bottom of the rectangular prism as p-polarized light. An incidentangle of the measuring beam was set at a smaller angle of resonance inthe TM₄ waveguide mode illustrated in FIG. 11. When the intensity of themeasuring beam reflected by the bottom of the prism was measured by asilicon photodiode for measurement, four waveguide modes were observedas illustrated in FIG. 10.

Outputs of the two photodiodes for reference and for measurement weresupplied to an electronic divider to calculate the ratio of the outputof the photodiode for measurement to the output of the photodiode forreference. The calculated ratio was supplied to an electric circuit togenerate an output signal indicative of a response of the opticalsensor. The result of the measurement revealed that for detectingtoluene in water, a detection limit was lower than the concentration at1 ppm and a 90% response time (a time required to reach a completeresponse) was within three minutes.

EXAMPLE 2

FIG. 14 shows how the response of the same optical sensor used inExample 1 varies as the concentration of toluene in water changes. Itshould be noted that the response from the optical sensor was measuredin the form of a change in reflectivity with respect to a change inconcentration of toluene. The measurement revealed that the opticalsensor linearly responded to toluene in water in concentration rangingfrom 0 to 20 ppm.

EXAMPLE 3

FIG. 15 is a graph showing how an output signal of the same opticalsensor used in Example 1 varies over time when the concentration oftoluene in water is 20 ppm. The optical sensor used in this measurementemployed a polymer thin film formed of a poly(ODMA-co-GLMA) layer of 107nm in thickness and a helium-neon laser at wavelength of 632.8 nm forthe light source. Surface plasmon resonance was observed as illustratedin FIG. 12 by the use of the polymer thin film of 107 nm in thickness.

An incident angle of a measuring beam, i.e., p-polarized light to thebottom of a prism was 60.82 which was rather a small incident angle inFIG. 12 illustrating the resonance coupling angle in the SPR method. Asthe output signal of the optical sensor was recorded as a function oftime, the output signal increased in response to toluene inconcentration of 20 ppm, and a 90% response time was approximately 3minutes.

EXAMPLE 4

The optical sensor having the structure illustrated in FIG. 1A was usedto measure toluene in concentration ranging from 0 to 300 ppm in waterin accordance with the FS-IER method. A detecting element had apoly(ODMA-co-GLMA) layer of 1.95 μm in thickness spin-coated on asilicon substrate, and was mounted in a housing as illustrated in FIG.1A. Water was introduced into the housing by suction. A laser diode foremitting light at wavelength of 670 nm was employed for a light source.Light from the laser diode was divided into a reference beam and ameasuring beam. The measuring beam was passed through a polarizing plateto be linearly polarized s-polarized light which was incident to thepolymer thin film at an incident angle of 80 through a glass window.

The intensity of the measuring beam reflected by the polymer thin filmwas measured by a silicon photodiode for measurement, while theintensity of the reference beam was measured by a silicon photodiode forreference. Outputs of these two silicon photodiodes were supplied to anelectronic divider to calculate the ratio of the output of the siliconphotodiode for reference to the output of the silicon photodiode formeasurement. The calculated ratio was used to generate an output signalby an appropriate electric circuit. FIG. 16 illustrates the reflectivitymeasured by the optical sensor in Example 4 which is plotted as afunction of the concentration of toluene in water. It can be seen fromFIG. 16 that the response of the optical sensor is not linear in therange of 0-300 ppm.

EXAMPLE 5

The optical sensor used to obtain the measurement results has the samestructure as the optical sensor illustrated in FIG. 6A. A sensingelement comprises a poly(ODMA-GLMA) thin film spin-coated on an SF11glass substrate in a thickness of 1 μm and a 90° glass prism mounted ona surface of the glass substrate, on which the poly(ODMA-GLMA) thin filmis not formed, by a refractive index matching oil. A He-Ne laser with awavelength of 632.8 nm is used as a light source, while photodiodes areused as first and second light detectors. As previously explained, alaser beam emitted from the light source is split by a beam splitterinto a probe beam and a reference beam. The probe beam is transformedinto s-polarized light by a polarizing plate and directed to be incidenton the prism at an incident angle of 45°, and the probe beam reflectedby the poly(ODMA-GLMA) thin film is supplied to the first lightdetector. The reference beam, in turn, is supplied directly to thesecond light detector. Signals representative of the intensities of theprobe beam and the reference beam are generated by the first and secondlight detectors, respectively, and supplied to an electronic circuit forproducing measurement results.

FIG. 17 is a graph representing the reflectivity of the optical sensorin response to 10-200 ppm toluene dissolved in water as a function oftime. It can be seen from this graph that the optical sensor of thisinvention has a high sensitivity and a fast response. Detection limitcould be around 1-2 ppm, and the 90% response time is less than 2minutes.

The relationship between the response of the sensing element and theconcentration of toluene in water is as illustrated in FIG. 18 whichreveals that the reflectivity of the sensing element linearly changes inproportion to the concentration of toluene in water.

EXAMPLE 6

Next, for examining how the response of the sensing element is relatedto the concentration of another organic substance in water, thereflectivity of the sensing element is measured while changing theconcentrations of benzene and p-xylene in addition to toluene. Theresults are shown in FIG. 19. The graph of FIG. 19 reveals that theresponse of the sensing element linearly changes in proportion to theconcentrations of the three kinds of organic substances, i.e., toluene,benzene, and p-xylene, and the sensitivity of the sensing element totoluene and p-xylene are 3.5 and 9.25, respectively, when thesensitivity of the sensing element to benzene is assumed to be one.

EXAMPLE 7

It is desirable that a polymer thin film has response to as many kindsof chemical substances under detection, i.e., DOC as possible fordetecting DOC in water. The optical sensor used in Example 1 having apoly(ODMA-co-GLMA) layer of 2 μm in thickness was used to measure alarge number of different kinds of DOC in water. The results of themeasurements are listed in the following table. In the table,“Sensitivity” is a value representing a change in reflectivity inpercent, and “time” represents a 90% response time.

TABLE 1 Concentration Time Organic compound (ppm) Sensitivity (Minute)toluene C₇H₈ 2 41.72 6 benzene C₆H₆ 2 10.34 4 chlorobenzene C₆H₅Cl 287.07 9 nitrobenzene C₆H₅NO₂ 2 23.28 1.5 -xylene C₈H₁₀ 2 126.86 15chloroform CHCl₃ 2 8.71 3 carbon tetrachloride CCl₄ 2 21.55 11.51,2-dichloroethane 2 6.03 1.5 ClCH₂CH₂Cl dichloromethane CH₂Cl₂ 2 1.38 1diethyl ether C₄H₁₀O 100 11.21 1.5 tetrahydrofuran O(CH₂)₄ 100 9.48 1acetone C₃H₆O 500 8.62 1 propanol C₃H₈O 500 6.03 1 methanol CH₄O 5000.00 acetic acid C₂H₄O₂ 500 3.45 <1 hydrochloric acid HCl 2000 25 <1sulfuric acid H₂SO₄ 2000 6.67 <1

It can be seen from Table 1 that the sensitivity and response time ofthe optical sensor vary depending on the kind of DOC to be detected. Amulti-channel system comprising an array of detecting elements or anarray of polymer thin films is therefore necessary to detect a largenumber of DOC in variety of industrial applications and environmentprotecting applications. In addition, known pattern recognitiontechniques such as matrix analysis, neural network analysis, or the likemay be applied together with such a multi-channel system in order toaccurately determine a total amount of DOC, the concentration of eachDOC, or the concentrations of a DOC group.

The pattern recognition techniques applicable to the detection ofchemical substances in water using the optical sensor of the presentinvention are described in an article entitled “Detection of HazardousVapors Including Mixtures Using Pattern Recognition Analysis ofResponses from Surface Acoustic Wave Device” by Susan L. Rosepehrsson etal. published in “Anal. Chem.”, 1088, 60, pp 2801-2811; an articleentitled “Development of Odorant Sensor Using SAW Response Oscillator

Incorporating Odorant-Sensitive LB Films and Neural-Network PatternRecognition Scheme” by Sang-Mok Chang et al, published in “Sensors andMaterials”, Vol. 1 (1995), pp 013-022; an article entitled“Polymer-based sensor arrays and multicomponent analysis for thedetection of hazardous organic vapors in the environment” by AndreasHierlemann et al. published in “Sensors and Actuators B” 26-27 (1995),pp 126-134; and so on.

As will be apparent from several embodiments and examples of the presentinvention described above in detail, the optical sensor according to thepresent invention is advantageous over the optical fiber sensor in thatit directly detects a change in thickness and/or refractive index of apolymer thin film resulting from interaction with a chemical substancein accordance with an optical approach such as the IER method, WGmethod, SPR method, or the like, so that even a trace of change inthickness and/or refractive index can be detected with a highsensitivity. In addition, since the polymer thin film can be readilyformed by an ordinary method such as spin coating, the optical sensoritself can be readily manufactured.

What is claimed is:
 1. An optical sensor for directly detecting achemical substance dissolved or dispersed in water comprising: at leastone sensing element, at least one light source unit and a first lightdetector, the sensing element including an optically transparentsubstrate and a polymer thin film capable of interacting with saidchemical substance dissolved or dispersed in water and formed on saidsubstrate and arranged in contact with said water, and wherein the lightsource unit is positioned on the substrate side of said sensing element,the first light detector is positioned on the substrate side of saidsensing element, and said substrate couples light from said light sourceunit to said polymer thin film at a predetermined angle, and functionsas light coupling means for coupling light reflected by said sensingelement to said first light detector, and said predetermined incidentangle is set at a value smaller than a critical angle of total internalreflection on the interface between said polymer thin film and saidwater and close to said critical angle.
 2. An optical sensor accordingto claim 1, wherein said chemical substance is organic carbon.
 3. Anoptical sensor according to claim 1, wherein said polymer thin film ismade of a material which enables measurement of said chemical substancein accordance with an IER method.
 4. An optical sensor according toclaim 1, wherein the thickness of said polymer thin film is selected tobe less than 10 μm.
 5. An optical sensor according to claim 1, whereinsaid polymer thin film is made of homopolymer or copolymer having arecurring unit represented by the following chemical formula (I):

where X represents —H, —F, —Cl, —Br, —CH₃, —CF₃, —CN, or —CH₂—CH₃; R¹represents —R² or —Z—R²; Z represents —O—, —S—, —NH—, —NR²′—, —(C═Y)—,—(C═Y)—Y—, —Y—(C═Y)—, —(SO₂)—, —Y′—(SO₂)—, —(SO₂)—Y′—, —Y′—(SO₂)—Y′—,—NH—(C═O)—; —(C═O)—NH—, —(C═O)—NR²′—, —Y′—(C═Y)—Y′—, or—O—(C═O)—(CH₂)n—(C═O)—O—; Y represents the same or different O or S; Y′represents the same or different O or NH; n represents an integerranging from 0 to 20; and R² and R²′ represent the same or differenthydrogen, a linear alkyl group, a branched alkyl group, a cycloalkylgroup, an un-saturated hydrocarbon group, an aryl group, a saturated orun-saturated hetero ring, or substitutes thereof, said R¹ notrepresenting hydrogen, a linear alkyl group, or a branched alkyl group,wherein X is preferably H or CH₃; R¹ is preferably a substituted ornon-substituted aryl group or —Z—R²; Z is preferably —O—, —(C═O)—O—, or—O—(C═O)—; R² is preferably a linear alkyl group, a branched alkylgroup, a cycloalkyl group, an un-saturated hydrocarbon group, an arylgroup, a saturated or un-saturated hetero ring, or substitutes thereof.6. An optical sensor according to claim 5, wherein said polymer thinfilm is made of a material comprising polymer or copolymer ofmethacrylic acid esters or acrylic acid esters.
 7. An optical sensoraccording to claim 1, wherein said substrate is a prism.
 8. An opticalsensor according to claim 1, wherein said substrate is planar and isformed with a grating on a predetermined position thereof.
 9. An opticalsensor according to claim 1, wherein said substrate is planar, and saidsensing element further includes a grating layer formed with a grating,said grating layer disposed between said substrate and said polymer thinfilm or on a surface of said surface on which said polymer thin film isnot formed.
 10. An optical sensor according to claim 1, wherein saidsubstrate is planar, and side-coupling is utilized as said lightcoupling means.
 11. An optical sensor according to claim 1, furthercomprising: a second light detector for directly receiving light fromsaid light source unit, and an electronic circuit coupled to receiveoutputs of said first light detector and said second light detector forcalculating the ratio of these outputs to generate a signal indicativeof the concentration of said chemical substance.
 12. An optical sensoraccording to claim 11, further comprising a housing for holding saidlight source unit, said first light detector, said second lightdetector, and said light coupling means in a predetermined positionalrelationship with respect to said sensing element.